The purpose of this study is to compare tamoxifen, versus a newer treatment, ARIMIDEX, which we already know to be well tolerated. The main objectives of this research study are to see whether using these drugs will shrink down tumours before surgery and to see if any shrinkage in the tumour affects the extent of surgery that is required.

Inclusion Criteria:

• Operable or potentially operable breast cancer
• Invasive breast cancer proven to be hormone-sensitive (ER+/PR+)
• Patients who might benefit from endocrine treatment prior to surgery
• Postmenopausal
• Signed written informed consent

Exclusion Criteria:

• Inoperable breast cancer
• Patient unwilling to undergo surgery
• Any reason why the patient may not be able to conform to study requirements
• Any previous treatment for breast cancer
• Previous Tamoxifen use as a part of breast cancer prevention trials
• Not willing to stop taking drugs that affect sex-hormones such as HRT
• Previous history of breast cancer
• Previous history of invasive malignancy within the last 10 years
• Any other severe disease that may prevent surgery, place the patient at risk, or influence the study results
• Treatment with an experimental drug within the last 3 months
• Risk of transmitting HIV, Hepatitis B or C